IKAROS Family Zinc Finger 2 (IKZF2) (also known as Helios) is one of the five members of the Ikaros family of transcription factors found in mammals. IKZF2 contains four zinc finger domains near the N-terminus which are involved in DNA binding and two zinc finger domains at the C-terminus which are involved in protein dimerization. IKZF2 is about 50% identical with Ikaros family members, Ikaros (IKZF1), Aiolos (IKZF3), and Eos (IKZF4) with highest homology in the zinc finger regions (80%+ identity). These four Ikaros family transcription factors bind to the same DNA consensus site and can heterodimerize with each other when co-expressed in cells. The fifth Ikaros family protein, Pegasus (IKZF5), is only 25% identical to IKZF2, binds a different DNA site than other Ikaros family members and does not readily heterodimerize with the other Ikaros family proteins. IKZF2, IKZF1 and IKZF3 are expressed mainly in hematopoietic cells while IKZF4 and IKZF5 are expressed in a wide variety of tissues. (John, L. B., et al., (2011), Mol. Immunol. 48:1272-1278; Perdomo, J., et al., (2000), J. Biol. Chem. 275:38347-38354.)
IKZF2 is believed to have an important role in the function and stability of regulatory T cells (Tregs). IKZF2 is highly expressed at the mRNA and protein level by regulatory T-cell populations. Knockdown of IKZF2 by siRNA has been shown to result in downregulation of FoxP3 and to impair the ability of isolated human CD4+CD25+ Tregs to block T-cell activation in vitro. Moreover, overexpression of IKZF2 in isolated murine Tregs has been shown to increase expression of Treg related markers such as CD103 and GITR and the IKZF2 overexpressing cells showed increased suppression of responder T-cells. IKZF2 has also been found to bind the promoter of FoxP3, the defining transcription factor of the regulatory T-cell lineage, and to affect FoxP3 expression.
Knockout of IKZF2 within FoxP3-expressing Tregs in mice has been shown to cause activated Tregs to lose their inhibitory properties, to express T-effector cytokines, and to take on T-effector functions. IKZF2 knockout mutant mice develop autoimmune disease by 6-8 months of age, with increased numbers of activated CD4 and CD8 T cells, follicular helper T cells and germinal center B cells. This observed effect is believed to be cell intrinsic, as Rag2−/− mice given bone marrow from IKZF2 knockout mice, but not bone marrow from IKZF2+/+ develop autoimmune disease. Direct evidence that IKZF2 affects regulatory T-cell function has been shown in the analysis of mice in which IKZF2 was deleted only in FoxP3 expressing cells (FoxP3-YFP-Cre Heliosfl/fl). The results showed that the mice also develop autoimmune disease with similar features as observed in the whole animal IKZF2 knockout. Moreover, pathway analysis of a CHIP-SEQ experiment has also suggested that IKZF2 is affecting expression of genes in the STAT5/IL-2Ra pathway in regulatory T-cells. This effect of IKZF2 loss was shown to be more apparent after an immune challenge (viral infection or injection with sheep's blood), and it was noted that after immune stimulation, the IKZF2 negative regulatory T cells began to take on features of effector T cells. (Getnet, D., et al., Mol. Immunol. (2010), 47:1595-1600; Bin Dhuban, K., et al., (2015), J. Immunol. 194:3687-96; Kim, H-J., et al., (2015), Science 350:334-339; Nakawaga, H., et al., (2016) PNAS, 113: 6248-6253)
Overexpression of Ikaros isoforms which lack the DNA binding regions have been shown to be associated with multiple human haematological malignancies. Recently, mutations in the IKZF2 gene, which lead to abnormal splicing variants, have been identified in adult T-cell leukemias and low hypodiploid acute lymphoblastic leukemia. It has been proposed that these isoforms, which are capable of dimerization, have a dominant negative effect on Ikaros family transcription factors which primes the development of lymphomas. IKZF2 knockout mutants that survive into adulthood do not develop lymphomas, supporting this hypothesis (Asanuma, S., et al., (2013), Cancer Sci. 104:1097-1106; Zhang, Z., et al., (2007), Blood 109:2190-2197; Kataoka, D., et al., (2015), Nature Genetics 47:1304-1315.)
Currently, anti-CTLA4 antibodies are used in the clinic to target Tregs in tumors. However, targeting CTLA4 often causes systemic activation of T-effector cells, resulting in excessive toxicity and limiting therapeutic utility. Up to ¾ of patients treated with a combination of anti-PD1 and anti-CTLA4 have reported grade 3 or higher adverse events. Thus, a strong need exists to provide compounds that target Tregs in tumors without causing systemic activation of T-effector cells.
An IKZF2-specific degrader has the potential to focus the enhanced immune response to areas within or near tumors providing a potentially more tolerable and less toxic therapeutic agent for the treatment of cancer.